1. Field of the Invention
The present invention relates to a radiation imaging apparatus that captures radiation images such as digital X-ray images, a radiation image processing apparatus, and an image processing method. In particular, the present invention relates to an offset correction technique of the radiation imaging apparatus.
2. Description of the Related Art
In recent years, as radiation imaging apparatuses for capturing radiation images such as images of X-rays transmitted through an object, radiation imaging apparatuses that directly convert a radiation image into digital signals in real time have become widespread. Especially, in order to examine the inside of a relatively large object such as a human body, large-area flat panel detectors (FPD) have been proposed. For example, an FPD is known in which minute radiation detectors are arrayed in a matrix on a substrate made of fused quartz. The radiation detectors are formed by layering a solid-state photodetector and a scintillator that converts radiation into visible light. The solid-state photodetector includes an amorphous semiconductor sandwiched between a transparent conductive film and a conductive film.
A radiation detector that uses a CCD or CMOS detector as a solid-state photodetector is also known. A radiation detector that directly detects radiation with a solid-state photodetector without using a scintillator is known as well.
Such FPDs detect the dose of radiation emitted during an arbitrary accumulation time period as the amount of electric charges. Therefore, in the case where electric charges that have no relevance to the emission of radiation exist in the detectors when capturing a radiation image, these electric charges are superimposed on the radiation image as noise and may trigger a decrease in the image quality of the radiation image. Examples of such electric charges that exist in the detectors and have no relevance to the emission of radiation include residual electric charges that remain after capturing a preceding radiation image based on the characteristics of the scintillators and solid-state photodetectors. Dark current, which is based on the electric charges generated mainly by the influence of temperature, may also occur in the solid-state photodetectors. Furthermore, there is fixed noise caused by defects unique to the solid-state photodetectors. When capturing a radiation image, residual electric charges and electric charges including dark current components increase in proportion to the accumulation time period for accumulating electric charges, and therefore the image quality is decreased. For this reason, upon capturing a radiation image, offset correction processing is executed to remove offset components that are attributed to residual electric charges and dark current electric charges accumulated during the image capture and to fixed noise.
In typical offset correction processing, an image obtained without emitting radiation (hereinafter referred to as an unirradiated image) is used as an offset correction image, and this offset correction image is subtracted from a radiation image. It should be noted here that an unirradiated image, which is obtained without emitting radiation as stated above, is typically obtained immediately before or immediately after capturing a radiation image. When it is necessary to obtain radiation images at high speed in succession as in the case of, for example, video capture, an offset correction image that has been prepared in advance may be used as it is difficult to obtain an unirradiated image between the operations of capturing the radiation images.
However, in typical FPDs, dark current electric charges are often unstable immediately after starting the driving of the detectors and immediately after emitting radiation. Also, residual electric charges that occur after the emission of radiation are known to change more drastically immediately after the emission of radiation is finished. Therefore, in order to execute stable offset correction processing, it is necessary to secure a certain time period between the start of the driving of the detectors and the image capture for a radiation image, or between the image capture for a preceding radiation image and the image capture for the next radiation image.
Furthermore, the use of an offset correction image that has been prepared in advance often fails to achieve adequate correction accuracy because dark current electric charges change due to the influence of, for example, the temperature of the detectors, imaging conditions, and deterioration of sensors over time.
Meanwhile, in order to improve the operability of radiation imaging apparatuses, it is desired that the radiation imaging apparatuses be capable of capturing the next radiation image as quickly as possible immediately after starting the driving and immediately after capturing a radiation image.
To achieve this object, Japanese Patent Laid-Open No. 2008-310466 discloses a technique to prepare a correction table showing the characteristics of decays of residual electric charges in association with imaging conditions and elapsed time periods, correct an offset correction image based on the correction table in association with imaging conditions and elapsed time periods, and use the corrected offset correction image. On the other hand, Japanese Patent Laid-Open No. 2010-233963 discloses a method for capturing a plurality of unirradiated images within a certain time period after a radiation emission start button is pressed down, and generating an offset correction image by fitting a temporal change in pixel values thereof using an approximate equation.
However, in the case where offset correction processing is executed based on the characteristics of decays that have been prepared in advance as disclosed in Japanese Patent Laid-Open No. 2008-310466, it is necessary to monitor the temperature during the image capture or similar statuses, and hence to set a new configuration therefor. Furthermore, when imaging conditions such as the temperature and the accumulation time period have changed since when the characteristics of decays were obtained in advance, it is necessary to generate an offset correction image through interpolation. This makes a decrease in correction accuracy unavoidable. Moreover, when the characteristics of decays in the detectors change over time, correction accuracy decreases significantly.
In view of the above, in order to improve the image quality of a radiation image, it is desired to generate a highly accurate offset correction image regardless of a time period that has elapsed since the start of the driving of the detectors, a time period that has elapsed since the last emission of radiation, and the amounts of residual electric charges and dark current electric charges.
Meanwhile, with regard to a method for calculating the characteristics of decays using a plurality of unirradiated images, which is disclosed in Japanese Patent Laid-Open No. 2010-233963, correction accuracy is maintained as long as a certain approximate equation holds. However, when the characteristics of decays are complex, a large number of unirradiated images are necessary to obtain a highly accurate approximate equation. As a result, a longer time period is required to perform the next image capture, thereby reducing the operability of radiation imaging apparatuses. Furthermore, as a temporal change in residual electric charges is influenced by the dose of radiation itself, it is difficult to apply the same approximate equation to, for example, pixels irradiated with a large dose of radiation and pixels irradiated with a small dose of radiation in an immediately preceding image capture. While Japanese Patent Laid-Open No. 2010-233963 also discloses a method whereby a plurality of approximate equations are applied and one of the approximate equations that yields the appropriate result is used, this method requires more arithmetic operations. As a result, due to various time delays, a further reduction in the operability of radiation imaging apparatuses is unavoidable.
For this reason, in the case where an offset correction image is generated using an approximate equation, it is desired to improve the operability of radiation imaging apparatuses by reducing the interval of image capture.